The present invention relates to a package for heating a plurality of food materials in a microwave environment. In particular, shielding material is often used in instances where two or more different food materials are to be simultaneously heated in a microwave oven, but one food material requires more heat than another The process of heating one food material more than another is referred to as "differential heating." Differential heating could be accomplished by employing a conductive shield, if certain problems could somehow be avoided. In the past, when attempts were made to use metal in a food package for use in a microwave oven, sparks and popping noises would occur when the microwave oven was turned on. This is commonly called "arcing", and has been a problem for many years--usually circumvented by avoiding use of metal in a microwave food package. Severe arcing could cause the package to burn.
Some other problems associated with the use of a metal or conductive shield include, in addition to arcing, scorching of the product or package, melting the package, resonant retransmission, retransmission on the edges of the shield, burning the package, localized overheating, standing waves, and apparent leakage of microwaves into the package. Applicant discovered that these problems all appear to be associated with resonances in the conductive shield. The present invention involves determining resonant lengths of microwave shielding material so that arcing and other problems associated with resonance in a food container for a microwave environment may be avoided.
The problem of arcing has plagued the art for many years where attempts have been made to use metallic shields to accomplish differential heating of food substances by microwave energy. Applicant discovered that arcing can be substantially eliminated by selecting an appropriate geometry for the metallic components of the food package. This discovery allows metal shields to be conveniently used to accomplish differential heating of food material without arcing and without damaging the microwave oven.
A need has long existed for a satisfactory arrangement which would permit a variety of food substances to be simultaneously heated in a microwave oven. But different food substances present significant problems in package design in order to achieve proper heating of the respective food substances. While convenience and packaging concepts indicate a need for different foods to be packaged together in a single container, this is oftentimes rendered impossible as a practical matter because one food substance typically requires more or less microwave heating as compared to another. For example, a need has existed for an arrangement which would permit a combination of food, such as ice cream and a sauce, to be exposed to the heating effects of a microwave oven in such a manner that the sauce would become hot while the ice cream remained substantially frozen. Strawberry shortcake with whipped cream, or pie and ice cream are other examples. Broccoli and cheese sauce is yet another example of a food combination that advantageously would benefit from a suitable package which permitted differential heating. Other examples of the need for differential heating of food substances will be readily apparent to those skilled in the art.
In the past, it has been the general belief that metal containers should not be used to heat and cook food in microwave ovens. This general belief was recently reiterated in U.S. Pat. No. 4,558,198, which issued on Dec. 10, 1985, to Levendusky et al. Levendusky et al.'s recent disclosure included the following discussion:
"It has been the general belief that metal containers should not be used to heat and cook food in microwave ovens. Bare metal containers can reflect the electromagnetic energy toward the magnetron (that supplies the energy to the oven cavity) and thereby damage the same. In addition, when bare metal is exposed in close proximity to the metal walls of a microwave oven, arcing between the container and oven walls occurs. For these reasons, the industry has generally advocated the use of plastic or cardboard containers to heat loads, e.g., foods, in microwave ovens."
Many others have recognized the problem of arcing in a microwave oven. For example, U.S. Pat. No. 4,122,324, issued to Falk, recognizes that slight imperfections in a metallic shielding film on a microwave food package may sometimes cause arcing. Falk says that arcing is "not uncommon" and can result from a scratch mark or even a small pin point in the metallic shielding film. Falk also discloses that other irregularities in the shape or edges of the shielding material can have the same effect because such irregularities, according to Falk, tend to concentrate the strength of the microwave field in those regions. Falk discloses that arcing presents a danger of fire because the temperatures generated in the region of the arc far exceed the flash point of the combustible material used to make the container or food package, which is typically made from thin cardboard, paper or the like.
While Falk recognizes the problem of arcing, Falk attempts to address the problem by coating the cardboard package to seal the package material from air and thereby minimize the tendency of the container to burn.
U.S. Pat. No. 4,439,656, issued to Peleg, recognizes arcing as a problem. Peleg addresses the problem by proposing an aluminum tray that is placed in a microwave transparent holder with a space between the tray and holder that is filled with water.
U.S. Pat. No. 3,854,021, issued to Moore et al., discloses a metal shield which lowers over part of a tray when the tray is inserted into the microwave oven. Moore et al. recognize that the shield distorts the microwave field in the oven and that arcing can result if the shield has sharp edges or is near the conductive wall of the oven. Moore et al. propose the use of Teflon tape on the lower edge of the shield to prevent arcing. The Moore et al. system for shielding is impractical for existing conventional microwave ovens because it would require substantial modification of an existing oven.
U.S. Pat. No. 4,558,198, issued to Levendusky et al., recognizes the significant problem of arcing. But Levendusky et al. say that a combination of four structures are needed to avoid arcing: (1) coating all surfaces of the tray with an organic coating at a very high film weight; (2) providing smooth curved wrinkle-free walls for the tray; (3) providing a round or oval shape in plan view such that there are no corners of the tray that are not curved or rounded with generous radii; and (4) providing a heat resistant plastic, microwave transparent dome or lid that covers the edges of the tray such that the edges are always physically separated and electrically insulated from the metal walls of the microwave oven. This reference actually teaches away from the present invention to the extent that Levendusky et al. instruct that all four structures are required to avoid arcing.
U.S. Pat. No. 4,351,997, issued to Mattisson et al., recognizes the problem of arcing. Mattisson et al. disclose that a traditional metallic tray is opaque to microwave radiation and is not suitable for use in microwave ovens which have no protection for the magnetron, because arcing may occur inside the oven cavity which may damage the magnetron. Mattisson et al. disclose a tray with aluminum foil laminate around the side walls of the tray.
U.S. Pat. No. 3,941,967, issued to Sumi et al., recognizes that aluminum foil may cause a "spark discharge" within a microwave oven. Sumi et al. disclose the use of an insulating body to prevent the occurrence of a spark discharge as a result of contact between the heating element and the inner wall of the oven.
Other proposals for use of metallic shielding to accomplish differential heating of food substances have been proposed. However, many older proposals have failed to even address the problem of arcing, much less solve that problem, and have not found significant commercial application to Applicant's knowledge. See, for example, U.S. Pat. No. 2,600,566, issued to Moffett, Jr.; and U.S. Pat. No. 2,714,070, issued to Welch. See also U.S. Pat. No. 4,081,646, issued to Goltsos. The difficulties involved in differentially heating various food substances in a single package led to the disclosure in U.S. Pat. No. 4,233,325, issued to Slangan et al., of a package which placed food substances in separate compartments sealed from one another. The wall between the compartments is punctured by a can opener or the like to mix the food substances after the food has been heated in a microwave oven and removed from the oven. Slangan et al. similarly ignore the problem of arcing, and fail to teach or suggest a solution to this problem.
None of the above-discussed references recognize the problem of resonance, and the other harmful effects associated therewith, such as localized overheating, scorching of the food material or the package, melting or burning of the package, edge overheating, retransmission, apparent leakage of microwaves into the package, standing waves, etc. By failing to recognize resonance as a problem, these references fall far short of addressing the problems solved by Applicant, and fall far short of obviously suggesting the solutions discovered by Applicant which are disclosed herein.
Because of the problem of resonance, and associated problems and effects including arcing and other problems enumerated above, metal shielding has found little use in commercial applications. Most microwave heating is still done in containers which are substantially transparent to microwave radiation and which contain no metal shielding.
Surprisingly, it has been found that the problems associated with resonance, including arcing, can be substantially eliminated and avoided while using a metal shield to accomplish differential heating of food material if the geometry of the shield is properly designed. Applicant discovered that the relationship between the wavelength of the microwave energy in the microwave oven and the dimensions of the shield could be properly controlled to avoid and to eliminate arcing, localized overheating, retransmitted fields, and other problems associated with resonance. Applicant has discovered that a metallic shield can be effectively used to accomplish differential heating of different food substances if the dimensions of the shield are intentionally selected in accordance with Applicant's teachings herein. Induced fields and parasitic currents which may occur in a metallic shield can be controlled if the teachings of this disclosure are followed.
Applicant also discovered that arcing and other problems can be eliminated by overlapping the ends of a metal shield in accordance with the teachings herein to effectively form an electrical damping arrangement. A practical shield may be typically formed by wrapping the metal shield around a container such that the ends of the shield overlap. Such overlapping is believed to in effect create capacitance that tends to damp voltages which would otherwise result in arcing. Overlapping tends to eliminate problems of arcing for half wavelength resonances, or odd multiples thereof. This is especially significant, because odd multiples of half wavelength resonances present the greatest potential for arcing. Overlapping therefore is an especially effective technique for eliminating arcing. The loop formed by wrapping the shield around the container in effect creates some inductance. A tuned circuit may be effectively formed from this combination of inductance and capacitance to control resonances in the metal shield.
The shield geometry should be designed to have nonresonant dimensions. It has been discovered that under circumstances where the shield becomes resonant, i.e., where the height, length, circumference, etc. of the shield is an integer multiple of a half wavelength, resonant voltages at the edges of the shield may be a prime cause of arcing. The discovery of the relationship between wavelength resonance of packaging materials and arcing has permitted metal shielding to be effectively used in packaging material while eliminating arcing. By eliminating the problems of arcing and other problems associated with resonance, metallic shields may now be used to allow a first food substance to be heated by microwaves while substantially reducing the exposure of a second food substance to the heating effects of the microwaves. Differential heating of two different food substances may thereby be accomplished with relative ease, without requiring substantial modifications to existing conventional microwave ovens.